1. Field of the Invention
The present invention relates to an image processing apparatus and method, adapted to reduce, when the color gamut of an output device is different from that of an input device, the color gamut of a color signal outside the output device color gamut to provide a color near to that in the input device. Also, the present invention relates to a color gamut conversion creating apparatus and method, adapted to create a color gamut conversion table for use to reduce a color gamut. Also, the present invention relates to a recording medium having recorded therein an image processing program in accordance with which a color gamut is reduced. Also, the present invention relates to a recording medium having recorded therein an color gamut conversion table creating program in accordance with which a color gamut conversion table for use to reduce a color gamut is created.
2. Description of the Related Art
These days, devices dealing with color image data are provided at lower and lower prices while their processing speeds have been higher and higher. In these situations, there has rapidly been in common use a variety of systems dealing with color images such as systems for transmitting and receiving color image data via networks such as Internet and the like, color desktop publishing systems for editing data including color images, etc.
There have been proposed various types of devices dealing with color images. However, many of them differ in color gamut they can cover (range of color reproduction) from one to another. Simple transfer of a color image between such devices of different types will result in reproduction of the color image in different colors from the original ones. Suppose for example that an image displayed on a monitor is printed out as a hard copy by a printer. In this case, if a color gamut the monitor covers is different from a one the printer covers, the color of an image printed out as a hard copy by the printer will be different from that of the image displayed on the monitor as the case may be.
As various systems dealing with color images have become popular, it has been demanded to attain a so-called device-independent color concept intended to reproduce a color image in the same colors at different types of devices included in the systems. The system to implement the device-independent color is generally called xe2x80x9ccolor management systemxe2x80x9d. As typical examples of this color management system, there are already available Colorsync included in Mac OS, ICM in Windows 98 and the like.
Referring now to FIG. 1, there is schematically illustrated the concept of the color management system in which physical colorimetric values of color signals in input and output devices are combined to implement a device-independent color. More particularly, as shown in FIG. 1, a color signal from an input device (such as video camera 61, scanner 62, monitor 63 or the like) is converted to a color signal in a device-independent color space (CIE/XYZ, CIE/L*a*b* or the like) on the basis of a device profile in which a color gamut conversion formula or color gamut conversion table is defined for each of the input devices. For an output device (monitor 63, printer 64 or the like) to output the color signal, the latter is converted to a color signal in a color space corresponding to the device on the basis of a device profile in which a color gamut conversion formula or color gamut conversion table is defined for each of the output devices.
Thus in the color management system, for conversion of an input device color signal to an output device color signal, a device-independent color is implemented by converting once the input device color signal to a color signal in a device-independent color space on the basis of a device profile. The xe2x80x9cdevice profilexe2x80x9d is a file in which a color gamut conversion formula or color gamut conversion table is defined. In other words, it is a file having stored therein a group of parameters calculated from relations between device color signals (RGB, CMYK or the like) and chromatic values (XYZ, L*a*b* or the like) measured by a colorimeter or the like.
Even if the color management system is applied, however, it is physically impossible for all the devices to reproduce the completely same because each of the devices has only a limited color gamut (range of color reproduction) which greatly differs from one device to another. That is to say, such differences in color gamut between all the devices are a barrier against implementation of the color management system.
The above difference in color gamut will further be described herebelow concerning a CRT monitor and printer. Normally, the CRT monitor reproduces a color by additive mixture of three color stimuli, namely, red (R), green (G) and blue (B), emitted from their respective phosphors on a face plate. Thus, the color gamut of the CRT monitor depends upon the types of the phosphors used on the face plate. On the other hand, the printer uses three color inks, namely, cyan (C), magenta (M) and yellow (Y) (or four color inks including black (K) in addition to the three color inks) to reproduce a color. That is, the color gamut of the printer depends upon the types of inks used therein. Further, the printer color gamut varies depending upon the type of a paper as an image recording medium, the gradation reproducing method, etc.
FIG. 2 shows a typical color gamut of CRT monitor and a typical color gamut of printer, integrated in the direction of L* and plotted in a plane a*-b*. Normally, the CRT monitor and printer color gamuts are different from each other as shown in FIG. 2. As seen from FIG. 2, the color gamut of the printer color is generally smaller than that of the CRT monitor, and especially in the green and blue color gamuts, the printer color gamut is extremely smaller than the CRT monitor color gamut. FIG. 3 shows the typical color gamut of CRT monitor and that of printer, plotted in a plane C*-L*. Since the peak of the chroma C* in the CRT monitor color gamut is away from that of the chroma C* in the printer color gamut in the direction of lightness L* as shown in FIG. 3, it is physically impossible for the printer to reproduce a color in an area of a high lightness and chroma displayed on the CRT monitor even in the domain of a hue in which there is not so large a difference between the CRT monitor and printer as in FIG. 2.
If the output device color gamut is smaller than the input device color gamut as in the above, the output device cannot reproduce all colors at the input device and the colors have to be processed in such a manner that they fall within the output device color gamut. For this purpose, all the colors have to be processed to fall within the output device color gamut while image information (gradation, tint, etc.) presented at the input device are being maintained. This process is generally called xe2x80x9ccolor gamut reductionxe2x80x9d. Especially, many printers are rather narrower in color gamut than other devices. So, for a printer to print out an, the color reproducibility often depends upon which color gamut reduction technique is employed.
It is most common that a color gamut is reduced in a common color space independent of any device, especially in a color space suitable for human visual sensation (for example, CIE/L*C*h color space). More particularly, a color gamut may be reduced after an input color signal is converted to a device-independent color signal as shown in FIG. 4. Otherwise, as shown in FIG. 5, when a device profile is created, a color gamut conversion formula or a color gamut conversion table may be defined taking also a color gamut reduction in consideration, and a color gamut reduction may be effected simultaneously with converting a color signal on the basis of the device profile.
Next, the color gamut reduction will further be described below:
The human color vision has three attributes including a lightness, chroma and hue. Generally, the color gamut reduction is effected in a color space based on these three attributes of the human color vision. The color space includes, for example, a CIE/L*C*h color space. The L*C*h is a polar coordinate to which L*a*b* and L*u*v* are converted. The L* indicates a lightness, C* indicates a chroma and h indicates a hue. These three attributes can be handled as independent parameters.
The techniques of color gamut reduction effected in such a color space are generally classified into three kinds: one-, two- and three-dimensional color gamut reductions.
In the one-dimensional color gamut reduction, only one of lightness, chroma and hue is changed. Normally in this method, only the chroma should preferably be reduced while the lightness and hue are kept constant as shown in FIG. 6 (as having been suggested by R. S. Gentile, E. Walowit and J. P. Allebach in xe2x80x9cA Comparison of Techniques for Color Gamut Mismatch Compensationxe2x80x9d, J. Imaging Tech., 16, pp. 176-181, (1990)).
In the two-dimensional color gamut reduction, two of lightness, chroma and hue are changed. Normally in this two-dimensional color gamut reduction, the chroma and lightness should preferably be reduced while the hue is kept constant. For the two-dimensional color gamut reduction, various techniques have been proposed. For example, E. G. Pariser proposed to reduce the chroma and lightness in the direction of (L*, a*, b*)=(50, 0, 0) with the hue kept constant as shown in FIG. 7 (in his xe2x80x9cAn Investigation of Color Gamut Reduction Techniquesxe2x80x9d, ISandT Symp. Elec. Prepress Tech.xe2x80x94Color Printing, pp. 105-107. (1991)). Also, the Japanese Unexamined Patent Application Publication No. 9-98298 has disclosed a technique that a color gamut should be divided for each hue and each divided color gamut be mapped in an optimum color gamut reducing direction as shown in FIG. 8.
In the three-dimensional color gamut reduction, lightness, chroma and hue are reduced. For such a three-dimensional color gamut reduction, the Applicant of the present invention has disclosed in the Japanese Unexamined Patent Application Publication No. 10-84487 a method of color gamut reduction in which each of three terms (lightness difference, chroma difference and hue difference) in a color difference formula is weighted (with a reduction factor) and the color gamut is reduced in the direction of a minimum color difference.
The three-dimensional color gamut reduction will further be described below:
A perceptive difference between two colors, quantitatively given is called xe2x80x9ccolor differencexe2x80x9d. On the assumption that two colors n the L*a*b* color space are (L1, a1, b1) and (L2, a2, b2), respectively, differences between attributes of each color can be given by the following equations (1-1) to (1-3), respectively, and a color difference xcex94E can be given by the following equation (1-4):
xcex94L*=L2xe2x88x92L1xe2x80x83xe2x80x83(1-1)
xcex94a*=a2xe2x88x92a1xe2x80x83xe2x80x83(1-2)
xcex94b*=b2xe2x88x92b1xe2x80x83xe2x80x83(1-3)
xcex94E={(xcex94L*)2+(xcex94a*)2+(xcex94b*)2}xc2xdxe2x80x83xe2x80x83(1-4)
To represent a color difference formula given by the equation (1-4) in a color space based on the three attributes of the human color vision, the chroma difference xcex94C* and hue difference xcex94H* are defined as given by the following equations (1-5) to (1-8):
C1={(a1)2+(b1)2}xc2xdxe2x80x83xe2x80x83(1-5)
C2={(a2)2+(b2)2}xc2xdxe2x80x83xe2x80x83(1-6)
xcex94C*=C2xe2x88x92C1xe2x80x83xe2x80x83(1-7)
xcex94H*=Sxc3x97{2xc3x97(C1xc3x97C2xe2x88x92a1xc3x97a2xe2x88x92b1xc3x97b2)}xc2xdxe2x80x83xe2x80x83(1-8)
However, when a2xc3x97b1xe2x89xa7a1xc3x97b2 in the equation (1-8), s=1, and when a2xc3x97b1 less than a1xc3x97b2, s=xe2x88x921.
At this time, the color difference xcex94E can be defined as given by the following equation (1-9):
xcex94E={(xcex94L*)2+(xcex94C*)2+(xcex94H*)2}xc2xdxe2x80x83xe2x80x83(1-9)
where xcex94L*, xcex94C* and xcex94H* are differences in lightness, chroma and hue, respectively, between two colors. The smaller the color difference xcex94E given by the equation (1-9), the smaller the perceptive difference between the two colors will be.
FIG. 9 shows an area where the color difference xcex94E given by the equation (1-9) is constant (this area will be referred to as xe2x80x9cconstant color difference areaxe2x80x9d hereinafter), plotted for some typical points in a plane a*-b*. As shown in FIG. 9, the difference between a color indicated with a mark xe2x80x9cxxe2x80x9d and a color plotted along a circle enclosing the mark xe2x80x9cxxe2x80x9d, namely, the color difference xcex94E given by the equation (1-9), will be constant at all points along the circle. Note that although the constant color difference area plotted in the plane a*-b* is indicated with the circle in FIG. 9, when the color difference area is considered three-dimensionally (a lightness L* is also included), it will be given as a spatial sphere.
In the color gamut reduction disclosed in the Japanese Unexamined Patent Application Publication No. 10-84487, the three terms (lightness, chroma and hue differences) included in the color difference formula given by the equation (1-9) are weighted with factors Kl, Kc and Kh (reduction factors), respectively, and then reduced in the direction of minimum color differences. Namely, on the assumption that the color difference formula is given by the equation (1-10), the color gamut is reduced for the color difference xcex94E given by the equation (1-10) to become minimum.
xcex94E={(xcex94L*/Kl)2+(xcex94C*/Kc)2+(xcex94H*/Kh)2}xc2xdxe2x80x83xe2x80x83(1-10)
When any of the reduction factors is given a large value, the reduction ratio for the attribute of a term corresponding to the large reduction factor becomes larger. This will be seen from FIG. 10. FIG. 10 shows that the color gamut reducing direction is changed by changing the reduction factor additionally put in the color difference formula. By changing the reduction factors Kl, Kc and Kh in this manner, it can be determined which one of the three attributes should be regarded as most important and reduced.
That is to say, for example, when one of the three reduction factors is given a large value, the color gamut reduction will be closer to the one-dimensional one. When two of the three reduction factors are made larger simultaneously, the color gamut reduction will be closer to the two-dimensional one. More specifically, as the reduction factor Kl is increased, the reduction ratio will be larger in the direction of lightness. As the reduction factor Kc is larger, the reduction ratio will be larger in the direction of chroma. Also, as the reduction factors Kl and Kc are made larger, mainly the lightness and chroma will be reduced while the hue is not so much changed. Namely, the color gamut reduction will be closer to the two dimensional one. When the reduction factors Kl, Kc and Kh are set to all one, the color difference will be equal to a one given by the equation (1-9).
FIG. 11 shows an example of the change of the constant color difference area in relation to the change of the reduction factor, and FIG. 12 shows another example of the change of the constant color difference area in relation to the change of the reduction factor. FIG. 11 shows a change of the constant color difference area when the reduction factor Kc is increased. In FIG. 11, a dotted-line circle indicates a constant color difference area given by the color difference formula represented by the equation (1-9), and a solid-line ellipse indicates a constant color difference area given by the color difference formula represented by the equation (1-10) in which the reduction factor Kc is increased. FIG. 12 shows a change of the constant color difference area when the reduction factor Kh is increased. In FIG. 12, a dotted-line circle indicates a constant color difference area given by the color difference formula represented by the equation (1-9), and a solid-line ellipse indicates a constant color difference area given by the color difference formula represented by the equation (1-10) in which the reduction factor Kh is increased.
As will also be seen from FIGS. 11 and 12, when the color difference formula is defined like the equation (1-10) using the reduction factors Kl, Kc and Kh, the constant color difference area can be changed by changing the reduction factors Kl, Kc and Kh. Thus, it can be determined which one of the three attributes should be regarded as most important and reduced.
Normally in the one- and two-dimensional color gamut reductions, the color gamut is reduced with the hue kept constant. For an image in colors of which many are outside the color gamut, however, the color gamut has to be reduced more in the direction of lightness or chroma. However, since the reduction of the color gamut of an image in the direction of lightness will reduce the contrast of the image, the more reduction of the color gamut in the direction of lightness will cause the whole image to lose a third dimension. On the other hand, the reduction of the color gamut in the direction of chroma will lower the definition of the image. So, if the color gamut is reduced more in the direction of chroma will cause the image to give a reduced impact. Especially, if the one- or two-dimensional color gamut reduction is applied to an image created by the computer graphic, namely, an image having an extremely high chroma and a third dimension, these features of the image will be lost to a considerable extent.
To apply a color gamut reduction to an image while maintaining such features thereof, the reduction ratio in the directions of lightness and chroma should be small while the hue is changed to some extent. This can be attained by the three-dimensional color gamut reduction.
However, the three-dimensional color gamut reduction is also disadvantageous in that a certain color will be changed too much in the direction of hue. This phenomenon will remarkably take place in the blue area. If the three-dimensional color gamut reduction is applied to an image including a blue area, the image will have only the blue area thereof appearing reddish. Note that this phenomenon will be a problem also in the one- and two-dimensional color gamut reductions as the case may be.
The cause for the blue area to appear reddish is that the hue of the blue area in a color space in which the color gamut is reduced has a considerable non-linearity. For example, in the CIE/L*a*b* color space, the hue line of the blue area is considerably bent. FIG. 13 shows a data prepared in the Munsell V3, plotted in the CIE/L*a*b* color space. The Munsell data was prepared on the basis of the human visual sensation, so that a Munsell data should be able to be linearly plotted radially in a color space which is based on the human visual sensation. In the CIE/L*a*b* color space, especially, in the blue area, however, the locus delineated by points derived from plotting of the Munsell data is a curve, from which it is known that in the CIE/L*a*b* color space, the hue line of the blue area is considerably bent. To improve the color reproducibility in an area in which the hue line is bent, the color gamut has to be reduced with the bending of the hue line taken in consideration.
It is therefore an object of the present invention to overcome the above-mentioned drawbacks of the prior art by providing an image processing apparatus and method, capable of reducing, when the color gamut of an output system is different from that of an input system, the color gamut of a color signal not falling in the output system color gamut to provide a color near to that in the input system.
It is another object of the present invention to provide a color gamut conversion table creating apparatus and method, adapted to create a color gamut conversion table for use to reduce a color gamut.
It is a still another object of the present invention to provide a recording medium having recorded therein an image processing program in accordance with which a color gamut is reduced.
It is a yet another object of the present invention to provide a recording medium having recorded therein a color gamut conversion table creating program in accordance with which a color gamut conversion table for use to reduce a color gamut is created.
In the color gamut reduction method disclosed in the Applicant""s Japanese Unexamined Patent Application Publication No. 10-84487, the color difference formula given by the equation (1-10) is used to reduce the color gamut in the direction of a minimum color difference xcex94E.
xcex94E={(xcex94L*/Kl)2+(xcex94C*/Kc)2+(xcex94H*/Kh)2}xc2xdxe2x80x83xe2x80x83(1-10)
On the other hand, according to the present invention, the color difference formula is defined as given by the following equations (2-1) and (2-2), and the color gamut reduction is made in the direction of a minimum color difference xcex94E given by the equation (2-1) or (2-2). By thus improving the color difference formula for use to reduce a color gamut, it is made possible to take the bending of the hue line in the color space in consideration, thereby permitting to reproduce a color with a higher accuracy.                               Δ          ⁢                      xe2x80x83                    ⁢          E                =                              {                                                            [                                      Δ                    ⁢                                          xe2x80x83                                        ⁢                                          L                      *                                        ⁢                                          xe2x80x83                                        ⁢                    Δ                    ⁢                                          xe2x80x83                                        ⁢                                          C                      *                                        ⁢                                          xe2x80x83                                        ⁢                    Δ                    ⁢                                          xe2x80x83                                        ⁢                                          H                      *                                                        ]                                ⁡                                  [                                                                                    Kll                                                                    K1c                                                                    Klh                                                                                                            Kcl                                                                    Kcc                                                                    Kch                                                                                                            Khl                                                                    Khc                                                                    Khh                                                                              ]                                            ⁡                              [                                                                                                    Δ                        ⁢                                                  xe2x80x83                                                ⁢                                                  L                          *                                                                                                                                                                        Δ                        ⁢                                                  xe2x80x83                                                ⁢                                                  C                          *                                                                                                                                                                        Δ                        ⁢                                                  xe2x80x83                                                ⁢                                                  H                          *                                                                                                                    ]                                      }                                1            /            2                                              (2-1)                                          Δ          ⁢                      xe2x80x83                    ⁢          E                =                                                                                                                        (                                                                        Δ                          ⁢                                                      xe2x80x83                                                    ⁢                                                      L                            *                                                                          Kl                                            )                                        2                                    +                                                            (                                                                        Δ                          ⁢                                                      xe2x80x83                                                    ⁢                                                      C                            *                                                                          Kc                                            )                                        2                                    +                                                            (                                                                        Δ                          ⁢                                                      xe2x80x83                                                    ⁢                                                      H                            *                                                                          Kh                                            )                                        2                                    +                                      (                                                                  Δ                        ⁢                                                  xe2x80x83                                                ⁢                                                                              L                            *                                                    ·                          Δ                                                ⁢                                                  xe2x80x83                                                ⁢                                                  C                          *                                                                    Klc                                        )                                    +                                                                                                                          (                                                                  Δ                        ⁢                                                  xe2x80x83                                                ⁢                                                                              C                            *                                                    ·                          Δ                                                ⁢                                                  xe2x80x83                                                ⁢                                                  H                          *                                                                    Kch                                        )                                    +                                      (                                                                  Δ                        ⁢                                                  xe2x80x83                                                ⁢                                                                              H                            *                                                    ·                          Δ                                                ⁢                                                  xe2x80x83                                                ⁢                                                  L                          *                                                                    Khl                                        )                                                                                                          (                  2          ⁢                      -                    ⁢          2                )            
where xcex94L* is a difference in lightness; xcex94C* is a difference in chroma; xcex94H* is a difference in hue; and Kl, Kc, Kh, Kll, Klc, Klh, Kcl, Kcc, Kch, Khl, Khc and Khh are predetermined constants, respectively, or functions of a lightness L*, chroma C* and hue h*, respectively.
By defining the color difference formula as given by the equation (2-1) or (2-2), the constant color difference area can freely be changed correspondingly to the hue or the like. This is shown in FIGS. 14 to 17. In FIGS. 14 to 17, a dotted-line ellipse indicates an example of the constant color difference area for which the color difference formula is defined as given by the equation (1-10). In FIGS. 14 and 15, a solid-line ellipse indicates an example of the constant color difference area of which the direction is changed by putting a term (xcex94L*, xcex94C*) in the color difference formula. In FIG. 16, a solid-line ellipse indicates an example of the constant color difference area of which the direction is changed by putting a term (xcex94C*, xcex94H*) in the color difference formula. In FIG. 17, a solid-line ellipse indicates an example of the constant color difference area of which the direction is changed by defining Kch as functions of the chroma C* and hue h.
By putting the term (xcex94L*xc2x7xcex94C*) in the color difference formula, the constant color difference area can be directed towards the directions of lightness L* and chroma C* while the hue his kept constant. Further, by defining Klc as a function of the lightness L*, it is also made possible to change an area having a high lightness L* in the direction of a lower lightness and an area having a low lightness L* in the direction of a higher lightness, as shown in FIG. 14.
Therefore, by defining the color difference formula as given by the equation (2-1) or (2-2), the constant color difference area can also be directed towards a point as shown in FIG. 15, for example. Thus, as in the two-dimensional color gamut reduction in which a color gamut is reduced in the direction of (L*,a*,b*)=(50, 0, 0), for example, the color gamut can be reduced in the direction of a certain point even when the three-dimensional color gamut reduction is adopted.
Also, by putting the term (xcex94C*xc2x7xcex94H*) in the color difference formula, the direction towards the achromatic axis of the constant color difference area can also be changed to another direction as shown in FIG. 16. The putting of the term (xcex94C*xc2x7xcex94H*) in the color difference formula is a very effective means for compensation of the bending of the hue line. Especially by defining Kch as a function of the hue h, it is also made possible to change the amount of compensation of the bending of the hue line for each hue. That is, by defining Kch as a function of the hue h to provide a larger amount of compensation for an area such as the blue area in which the hue line is much bent while providing a small amount of compensation for an area in which the hue line is not much bent, the color gamut can be reduced correspondingly to a bending of the hue line.
By defining Kch as functions of the chroma C* and hue h, the color difference formula can also be defined for the constant color difference area to extent along the bent hue line as shown in FIG. 17. This is very effective for compensation of the blue area in the color gamut reduction. By defining Kch as functions of chroma C* and hue h, it is made possible to solve the problem that when a color gamut is reduced, the direction of the constant color difference area is changed excessively in the direction of the hue in the blue area.
According to the present invention based on the principle having been described in the foregoing, there is provided an image processor adapted to convert, for outputting, an image from a predetermined input device to an image corresponding to the color gamut of a corresponding output device, the image processor including means for reducing, when the output device color gamut is different from the color gamut of the input device, the color gamut of a color signal outside the output device color gamut in the direction of a minimum value of the color difference formula given by the above equation (2-1) or (2-2).
According to the present invention, there is also provided an image processing method of reducing, if the color gamut of an output device is different from that of an input device when converting, for outputting, an image from a predetermined input device is converted to an image corresponding to the color gamut of a corresponding output device, the color gamut of a color signal outside the output device color gamut in the direction of a minimum value of the color difference xcex94E given by the equation (2-1) or (2-2).
According to the present invention, there is also provided a color gamut conversion table creating apparatus adapted to create a color gamut conversion table to which reference is made when converting, for outputting, an input color signal from a predetermined input device to a color signal corresponding to the color gamut of a predetermined output device, the apparatus including a color gamut conversion table creating means for reducing the color gamut of any of colors inside the color gamut of an input device color gamut, not inside the color gamut of an output device, in the direction of a minimum color difference xcex94E given by the equation (2-1) or (2-2) to have the color correspond to a color inside the output device color gamut, and creating, on the basis of the result of the correspondence, a color gamut conversion table showing relations between input device color signals and output device color signals.
According to the present invention, there is also provided a color gamut conversion table creating method adapted to create a color gamut conversion table to which reference is made when converting, for outputting, an input color signal from a predetermined input device to a color signal corresponding to the color gamut of a predetermined output device, the method including a step of reducing the color gamut of any of colors inside the color gamut of an input device color gamut, not inside the color gamut of an output device, in the direction of a minimum color difference xcex94E given by the equation (2-1) or (2-2) to have the color correspond to a color inside the output device color gamut and create, on the basis of the result of the correspondence, a color gamut conversion table showing relations between input device color signals and output device color signals.
According to the present invention, there is also provided a recording medium having recorded therein an image processing program in accordance with which an image from a predetermined input device is converted to an image corresponding to the color gamut of a predetermined output device, the program being such that when the color gamut of the output device is different from that of the input device, the color gamut of a color signal outside the output device color gamut is reduced in the direction of a minimum color difference xcex94E given by the equation (2-1) or (2-2).
According to the present invention, there is also provided another recording medium having recorded therein a color gamut conversion table creating program in accordance with which there is created a color gamut conversion table to which reference is made when converting, for outputting, an input color signal from a predetermined input device to a color signal corresponding to the color gamut of a predetermined output device, the program being such that the color gamut of any of colors inside the color gamut of an input device color gamut, not inside the color gamut of an output device, is reduced in the direction of a minimum color difference xcex94E given by the equation (2-1) or (2-2) to have the color correspond to a color inside the output device color gamut and create, on the basis of result of the correspondence, a color gamut conversion table showing relations between input device color signals and output device color signals.
These objects and other objects, features and advantages of the present intention will become more apparent from the following detailed description of the preferred embodiments of the present invention when taken in conjunction with the accompanying drawings.